Suppressing a rotation by a RBE3-element

I'm trying to optimize a tunnel model in Siemens NX 11.0. I'm applying the loads stepwise in time. Between 0s. and 1s., only thrust force is acting on the tunnel. However, a small clockwise rotation appears in the results and I have no idea where it comes from.

In order to suppress the small rotation, I tried to add a RBE3-connection between nodes at the circumference of the tunnel (source) and a node at the center of the tunnel ring (target). I'm supposing to constraint DOF1, DOF2 and DOF6 of the central node, as no rotation around the longitudinal z-axis are allowed.

Still I have errors in my model set-up. I tried different ways to define the boundary conditions but none of them has worked. In attachment, you can find the model.